In a typical communication system, it is often desirable to modulate a carrier with baseband signals to be transmitted, where the carrier frequency is many times higher than the frequency of the baseband signals to be transmitted. This modulation scheme, typically, employs a multiplier that multiplies the baseband signal with an output signal of a local oscillator. The resulting signal, in this context, referred to as a passband signal, comprises a version of the baseband signal shifted into the carrier frequency range.
One type of modulation scheme employed in many communication systems is referred to as M-ARY phase shift keying, where M is a positive integer. This type of signal modulation is well-known and described, for example, in Information, Transmission, Modulation and Noise, by Mischa Schwartz, McCraw Hill (3rd ed.) 1980. Basically, phase shift keying is a signal encoding scheme, where one or more data bits may be represented by a symbol signal having a given amplitude and a given phase. One type of M-ARY phase shift keying is Quadrature Phase Shift Keying or QPSK. According to this scheme, two binary bits may be represented by a symbol signal having different phases approximately 90.degree. apart. FIG. 2 illustrates a signal constellation for a QPSK modulation scheme. For example, data bits "00" correspond to a symbol signal that may be represented by phasor 10, having a given amplitude and "zero" phase. Data bits "01" correspond to a symbol signal that may be represented by phasor 12, having substantially the same amplitude as phasor 10, and a phase shift of .pi./2 radians. Data bits "10" correspond to a symbol signal that may be represented by phasor 14, having a phase shift of .pi. radians. Likewise, data bits "11" correspond to a symbol signal that may be represented by phasor 16, having a phase shift of 3.pi./2 radians.
Conventional communication receivers have a substantially complex arrangement for M-ARY phase shift keying demodulation. Typically, these systems include a frequency tracking module to determine or recover the carrier signal of the incoming transmitted signal, a multiplier to convert the incoming signal from a passband to a baseband frequency region, and filters for smoothing noise signals. The design arrangement of conventional M-ARY demodulators, therefore, typically includes complex analog circuitry. This arrangement also typically includes digital circuitry for processing the incoming recovered data bits. Although, it is typically difficult to implement an integrated circuit M-ARY demodulator that includes both analog and digital circuitry, providing such a demodulator on a chip may provide advantages in terms of convenience and cost. Hence, there is a need for a M-ARY demodulator that can be implemented on a single integrated circuit chip.
Furthermore, the conventional process of determining the carrier signal of an incoming transmitted signal is substantially time consuming. The time required to determine the carrier signal may not be acceptable for burst mode communications. In a typical burst mode communication system, like time division multiple access (TDMA), for example, a plurality of transmitters send signals over the same channel to one receiver. Each transmitter sends signal bursts or packets of information that include a preamble portion that is used by the receiver to identify the transmitter and to determine the carrier signal for the duration of the burst. The receiver continually receives these bursts of information, which are spaced apart by a substantially short idle period. Due, at least in part, to the relatively long time involved to recover a carrier signal, and furthermore, due to the complexity of a conventional circuit configured to multiply and filter the incoming signals to convert them into the baseband region, conventional demodulators may not be appropriate for high bit-rate burst mode communications.
Thus, a need exists for a M-ARY demodulator that is suitable even for a high-bit rate burst mode communications environment and that has an integrated circuit arrangement.